1. Field of the Invention
The present invention relates to a multilevel signaling system for transmitting signals across a multiconductor transmission line.
2. Background
Various techniques and systems are available for transmitting data between a source and a destination. When data is electrically transmitted across a conductor, a particular signaling technology or protocol is utilized. A set of symbols may be associated with specific signaling technologies. The symbols are used to encode the data into various electrical patterns on the transmission line conductors such that each symbol may be distinguished from other signals by analyzing the electrical pattern on the conductors. The conductors used to transmit data include wires, cables, traces on printed circuit boards, conductors embedded within a substrate, and various other conductive materials.
In certain data transmission systems, the conductors are treated as transmission lines and analyzed by considering various electrical and electromagnetic wave properties and characteristics. In these systems, the signaling technology may include the coupling of signal drivers, signal receivers, conductors, and termination devices.
A particular type of data transmission system transmits signals differentially. For example, FIG. 1 illustrates a known differential transmission system using a pair of conductors. A differential driver 10 receives data on input 12 and transmits differential signals across conductors 14 and 16. Conductor 14 is coupled to the non-inverting output of driver 10 and conductor 16 is coupled to the inverting output of driver 10. A differential receiver 18 (also referred to as a differential comparator) receives the differential signals from conductors 14 and 16, and generates an output on line 20. Conductor 14 is coupled to the non-inverting input of receiver 18 and conductor 16 is coupled to the inverting input of receiver 18. A pair of terminating resistors 22, 24 are coupled between conductors 14 and 16, and a terminating voltage V.sub.term.
In the system of FIG. 1, the pair of conductors 14, 16 are capable of transmitting two symbols representing a binary zero or binary one. The data provided to driver 10 represents one of two possible symbols; e.g., a binary zero or one. Driver 10 transmits a particular signal pattern on conductors 14, 16 based on the input data provided to the driver. For example, when a binary zero is the input data, driver 10 generates a logic low signal on its non-inverting output which is coupled to conductor 14. Driver 10 also generates a logic high signal on its inverting output which is coupled to conductor 16. Conversely, when a binary one is the input data, driver 10 generates a logic high signal on its non-inverting output and generates a logic low signal on its inverting output. Thus, the polarity of the outputs from differential driver 10 are always opposite one another. The output polarity is controlled by the input signal applied to driver 10.
Differential driver 10 may be a current mode driver which produces output currents (i.sub.0 and i.sub.1) in response to the input provided. The value of v.sub.0 is defined as v.sub.0 =V.sub.term -i.sub.0 R.sub.t. Similarly, the value of v.sub.1 is defined as v.sub.1 =V.sub.term -i.sub.1 R.sub.t. Receiver 18 compares the voltage levels on its two inputs and generates the data output signal corresponding to the input provided to driver 10.
The differential signaling system illustrated in FIG. 1 requires two conductors 14, 16 to transmit a single bit of data. Therefore, this method results in an inefficient use of data interconnect resources (number of conductors =2.times.number of bits transmitted). Certain applications may require a more efficient use of interconnect resources in a differential transmission system. Thus, it is desirable to provide a system having the advantages provided by differential signaling, but without the inefficient ratio of the number of conductors to the number of bits transmitted.